The goal of this project is to identify genetic markers useful in early detection of medullary thyroid carcinoma. Preliminary data indicate a higher percentage of chromosomal aberrations and deletions in patients with medullary carcinoma of the thyroid. This project will cover patients with this disease as well as their first and second degree relatives. These findings will be correlated with a combined calcium pentagastrin stimulation test for detection of medullary carcinoma of the thyroid. Similar studies will be done in 101 sporadic cases of medullary carcinoma of the thyroid and their first degree relatives for comparison with the familial type of the disease.